The RFID systems have already been prevailed. A single RFID system allows a reader/writer to read information stored in a wireless tag. Specifically, the reader/writer is configured to transmit roughly 1 W signals using a wireless line of the UHF band (of 860-960 MHz), whereas the wireless tag is configured to receive the signals and send response signals back to the reader/writer. The wireless frequency band, used in Japan, ranges from 952 MHz to 954 MHz. Read range is herein roughly 3-10 m, although depending on: the antenna gain of a wireless tag; the operating voltage of a wireless IC chip; the antenna gain of a reader/writer; and the surrounding environment.
The wireless tag includes an antenna and a wireless IC chip (of a square type having roughly 0.5 mm sides) to be connected to a power supply contact of the antenna.
In the wireless tag, an antenna pattern is formed on a translucent film sheet by means of printing, etching and the like. The wireless IC chip is connected to the power supply contact of the antenna although no special matching circuit is mounted on the power supply contact.
As illustrated in FIG. 1, a wireless IC chip (hereinafter simply referred to as “an IC chip”) is allowed to be equivalently expressed with a parallel circuit having an internal resistor Rc (of e.g., 1700Ω) and a capacitance Cc (of e.g., 1.0 pF). Further, an antenna is allowed to be equivalently expressed with a parallel circuit having a radial resistor Ra (of e.g., 2000Ω) and an inductance La (of e.g., 30 nH). When the IC chip and the antenna are connected in parallel, resonance occurs between the capacitance Cc and the inductance La and impedances are accordingly matched at a desired resonance frequency fo (of e.g., 953 MHz). Accordingly, electric power received by the antenna is maximally provided to the IC chip. It is herein noted that the aforementioned resonance frequency fo is expressed by “1/(2π*(La*Cc)1/2”.
When wireless tags are attached onto nonconductive members (or nonmetal members) such as cardboard members or plastic members, the dipole antennas are often used as the wireless-tag antennas. A single dipole antenna may include a meander-shaped dipole section, and the entire length thereof may be roughly 140 mm corresponding to λ/2. In this case, a single wireless tag may have a size of 10 cm by 2 cm.
When being attached onto a given disc-shaped recording medium (CD, DVD, etc), the wireless tag with the size of roughly 10 cm by 2 cm overlaps an annular metal section for a data recording purpose (hereinafter referred to as “a first metal section”) of the recording medium in a plan view. Specifically, the annular first metal section of the recording medium has an inner radius of ø36-40 mm and an outer radius of roughly 120 mm (roughly equal to the outer diameter of the entire recording media). Therefore, the wireless tag of the size of 10 cm by 2 cm overlaps the first metal section when being attached onto the recording medium. Under the condition, wireless communication is completely blocked between the wireless tag and the reader/writer by means of a shield effect of the first metal section.
In view of the above, Japan Laid-open Patent Application Publication No. JP-A-2007-166573 proposes a wireless tag preferable for attachment onto a disc-shaped recording medium (CD, DVD, etc.). The wireless tag is configured to be attached onto a disc-shaped recording medium while a monopole antenna thereof is disposed on a nonconductive member (specifically, a translucent plastic section) arranged on the inner radial part of the recording medium. Further, electromagnetic coupling is configured to occur between the wireless tag and a first metal section of the recording medium when an annular flat ground section of the wireless tag partially overlaps the first metal section of the recording medium in a plan view. With the configuration, the first metal section of the recording medium is allowed to be assumed as a ground at a high frequency. Therefore, wireless communication is fully executable between the wireless tag and a reader/writer.
In the aforementioned wireless tag of the type proposed so far, the disc-shaped recording medium is assumed to include the first metal section having an inner diameter of ø36-40 mm. Further, a monopole antenna is assumed to be formed within the first metal section of the above range. However, the disc-shaped recording media of another type have been introduced in the market in these years. A single recording medium of the type includes a metal section for a non-data-recording purpose (hereinafter referred to as “a second metal section”) disposed radially inwards of the first metal section. It is confirmed that the inner diameter of the second metal section ranges from ø20 mm to ø27 mm. FIG. 2 illustrates the structure of a disc-shaped recording medium including a first metal section and a second metal section.
Such a recording medium, including a metal section with a small inner diameter, is allowed to include a narrow region made of only a nonconductive member on the inner radial part thereof. Therefore, it is difficult to mount a monopole antenna thereon. When being formed in the narrow annular region, for instance, a monopole antenna of a well-known type is assumed to include a double-circled/triple-circled monopole section within the narrow annular region. Alternatively, the monopole antenna is assumed to include a meander-shaped monopole section having a narrow line width within the narrow annular region. In the aforementioned monopole-antenna forming method, however, significant reduction in read range is unavoidable between the monopole antenna and the reader/writer.